1. Field of the Invention
The invention relates to a device with a circuit arrangement comprising amplifier means for the amplification of signals, which amplifier means comprise an amplifier output, on which the amplifier means produce amplified signals, and safety means for limiting the output current of the amplifier means, the circuit arrangement further comprising filter means connected to the amplifier output of the amplifier means, which filter means are adapted to extract undesired signal components from the amplified signals and comprise a series arrangement of a capacitor and a resistor.
2. Description of the Related Art
Such a device of the type defined in the opening paragraph is known, for example, from the Data Book entitled "Semiconductors for Radio and Audio Systems TDA 1386T to TSA 6060" published by the Applicant in 1995. The Data Book discloses such a device in the form of an application circuit for an integrated device (IC) having the type designation TDA1514A.
For the processing of signals, in particular for amplifying them, the known device comprises a circuit arrangement with amplifier means, and a loudspeaker for converting amplified signals into sound waves and for reproducing the sound waves, the amplifier means of the circuit arrangement having an amplifier output connected to one terminal of the loudspeaker, which has a second terminal connected to a reference potential. In the known device the loudspeaker constitutes the load connected to the amplifier means.
In the event of a short-circuit of the output, the amplifier output is connected to the reference potential terminal, and the amplifier means will supply a comparatively large output current, as a result of which the power dissipation in the amplifier means increases strongly and, consequently, the temperature of the amplifier means rises strongly. The amplifier means of the known device include safety means which limit the output current and hence the power dissipation of the amplifier means in the case of such a short-circuit of the output, which precludes damage to the amplifier means as a result of an excessive power dissipation and a consequent excessive temperature rise in the amplifier means or an excessive temperature of the amplifier means. The safety means of the amplifier means in the known device are formed by power-increase safety means, which detect a rapid rise of the power dissipation towards a critical power dissipation, and by temperature safety means, which detect a critical temperature of the amplifier means, and they limit the output current of the amplifier means when either one of the power-increase safety means and the temperature safety means detect a critical value.
The circuit arrangement of the known device has filter means connected to the amplifier output of the amplifier means, which filter means are adapted to remove undesired high-frequency signal components from the amplified signals. Particularly signal components of amplified signals having a high frequency and lying outside a useful frequency range of the device, which range extends from approximately 0 Hz to 20 kHz, can have an adverse effect because such undesired high-frequency signal components of frequencies above the useful frequency range of the device may give rise to undesirable disturbances. If the circuit arrangement would have no filter means the undesired high-frequency signal components would not be removed from the amplified signals, which would give rise to instabilities in the circuit arrangement, in which case the circuit arrangement would also supply amplified signals to the loudspeaker when the circuit arrangement receives no signals to be amplified, which would result in undesirable disturbances. In the known device such undesired high-frequency signal components of the amplified signals are removed by the filter means, which comprise a series arrangement of a capacitor and a resistor having a low resistance of only a few ohms, the capacitor of the filter means being connected directly to the amplifier output and the resistor of the filter means being connected to the reference potential terminal. In this way the filter means are connected in parallel with the loudspeaker to the amplifier output of the amplifier means. Among those versed in the art, such filter means are referred to as a Boucherot circuit.
Such a device of the type defined in the opening paragraph has the problem that in the event of a fault condition the resistor of the filter means receive a comparatively large output current from the amplifier means, in which case the resistor of the filter means is subjected to a excessive load. Such a fault condition occurs, for example, if owing to, for example, excessive aging, a production fault which manifests itself at a later stage, or the occurrence of an overvoltage the capacitor of the filter means breaks down and then constitutes a short-circuit, in which fault condition the resistor of the filter means is connected to the output of the amplifier means via the short-circuit formed by the capacitor. The safety means of the amplifier means then do not limit the output current of the amplifier means, because the output is not short-circuited and, as a consequence, the power dissipation in the amplifier means is smaller than the critical power dissipation for the amplifier means, although a comparatively large output current is produced owing to the defective and consequently short-circuited capacitor of the filter means. In addition to the undesired high-frequency but low-power signal components already appearing across the resistor of the filter means in normal operation, this also results in a substantial portion of the low-frequency high-power signal components of the amplified signal, which in normal operation appear only across the load, and hence a substantial portion of the comparatively large output current of the amplifier means appearing across the resistor of the filter means in said situation, which resistor has only a small resistance value. If in such a known device the resistor of the filter means is a commercially available low-cost resistor, this resistor will be damaged by the comparatively large output current applied to it by the amplifier means, as a result, of which this resistor would assume an impermissibly high temperature and could catch fire. As a consequence, the whole device could then catch fire. For this reason it is known and also prescribed by safety regulations to use a safety resistor for the resistor of the filter means. When a given temperature is reached, which is lower than the impermissibly high temperature, such a safety resistor passes into an irreversible no-load condition, thus precluding any safety risk presented by an impermissibly high temperature for the safety resistor forming the resistor of the filter means. However, such safety resistors are comparatively expensive, i.e. even substantially more expensive than commercially available low-cost resistors, so that their use leads to an undesirable rise in cost of a device of the type defined in the opening paragraph.